1. Field of the Invention
The present invention relates to a data transmission method in a mobile communication system.
2. Description of Related Art
Since an Advanced Mobile Phone Service (AMPS) was started in the United States at the beginning of 1980s, and since an automobile telephone service was started in major cities of Japan, the number of cellular mobile users has so far multiplied dramatically. Accordingly, mobile communication systems were required to have a large capacity enough to accommodate a great number of users and analog cellular services using a large-capacity backbone network were launched in 1988. These analog cellular services are called the first generation (1G). Thereafter, digital cellular mobile telephone services based on Time Division Multiple Access (TDMA), positioned as the second generation, were started in the early 1990s. In Europe, a Global System for Mobile Communications (GSM) was started in 1991. In Japan, a Personal Digital Cellular (PDC) service was started in 1993. In the United States, a Digital AMPS (D-AMPS) (IS-54) was launched in 1993. These 2G systems led to the current prosperity of mobile communications. Somewhat later, an IS-95 (cdmaOne) system which adopted Code Division Multiple Access (CDMA) was put into practical use, and the IS-95 service was launched in South Korea and the United States in 1996 and also in Japan in 1998. Some calls the IS-95 based systems the second and a half generation (2.5G) to differentiate them from the second generation (2G) TDMA systems. In October, 2001, a W-CDMA service, which is a system of IMT-2000, positioned as the third generation (3G), was launched in Japan.
Active efforts toward the next generation mobile communications, the fourth generation mobile communications (4G) are also being made and a goal of realizing mobile communications in a 100 Mbps cellular environment has come to be recognized widely. The communication rate of 100 Mbps is about 100 times as faster as the IMT-2000 and it is an important challenge how to achieve such a high transmission rate in as narrow a frequency band as possible in order to make effective use of finite resources of radio frequencies.
Spectrum efficiency, the term which will be used hereinafter, is defined as follows. Through the use of total bit rate R per cell (or sector if the system is divided into sectors) and system frequency bandwidth W which is used in expanding service area on to surface, a ratio of R/W is referred to as the spectrum efficiency. If a 1-Hz frequency band is assigned to a mobile communication system, this spectrum efficiency corresponds to a maximum bit rate allowed for a user who occupies one cell or sector. Enhancing the R/W ratio means enhancing the maximum bit rate that the system can provide to users.
When viewing the above mobile communication systems which have heretofore been adopted by a measure of the spectrum efficiency, it will be understood that the spectrum efficiency R/W has been improved more as the generation advances, as is shown in FIG. 16. Also, it is indicated here that the spectrum efficiency improvement so far made to the mobile communication systems has a close relation to reduction in Eb/No required for the mobile communication systems. Eb denotes energy required to transmit one bit of data to be communicated and No denotes noise power density in the frequency band. It is reasonable that the reduction in the required Eb/No was achieved mainly by the following technologies: digitizing for 2G, CDMA and Viterbi code adopted for 2.5G, and Turbo code adopted for 3G.
Further spectrum efficiency improvement is expected for 4G to be made by new technologies which have not been utilized positively in the 3G and earlier mobile communication systems. One of such technologies is utilizing adaptive array antennas. A signal transmission method using the adaptive array antennas is depicted in FIG. 17. At the transmitting end, data to transmit is channel encoded 100 and modulated 100 and the modulated transmit signal is multiplied by M units of complex weight factors, and thereby transmit beams are generated (102). The beams are transmitted from M units of transmitting antennas (103-1 through 103-M). Each factor by which the transmit signal is multiplied during the beam forming 102 and the positions of the transmitting antennas (103-1 through 103-M) determine the pointing directions of the transmit beams. By controlling the pointing directions, the transmit beams can be configured to make radio beam emission power strongest in the direction toward the target receiver. At the receiving end, signals received by N units of receiving antennas (104-1 through 104-N) which are arranged in an array are multiplied by complex weight factors and added and combined (105). Each factor by which the received signals are multiplied during the beam forming 105 and the positions of the receiving antennas (104-1 through 104-N) determine the pointing directions of receiving beams. By controlling the pointing directions, the receiving beams can be configured to make radio beam receiving power strongest in the direction from the particular transmitter and relatively suppress radio beam receiving power in other directions. Thereby, the desired radio beam is made strong against interference beams.
Another one of the above-mentioned new technologies is utilizing Multiple-Input Multiple-Output (MIMO) propagation channels. A signal transmission/reception method using the MIMO channels is depicted in FIG. 18. Data to transmit is channel encoded 100 into M units of different signals which are then respectively modulated by modulators (101-1 through 101-M) and transmitted from a plurality of transmitting antennas (103-1 through 103-M) which are arranged in an array. The M units of transmit signals are mixed complexly through the propagation channels and come to the receiving end. At the receiving end, signals received by N units of receiving antennas (104-1 through 104-N) which are arranged in an array are multiplied by a complex matrix of M rows by N columns (108), and thereby M units of signals are obtained. By configuring the complex matrix such that the signals mixed through the above propagation channels are separated each other, the signals corresponding to the signals modulated by the modulators 101-1 through 101-M can be output from the MIMO receiver 108. Then, the M units of signals are respectively demodulated by demodulators 106-1 through 106-M and decoded by a channel decoder 107, and thereby received data is obtained. In the above-described method, the M units of different signals can be transmitted in parallel on the channels of the same radio frequency and communication with a high spectrum efficiency is considered achievable.
However, in fact, spectrum efficiency improvement to a great degree cannot be expected by applying the above-discussed technologies to mobile communication. There is a theoretical limitation to the spectrum efficiency, which is called a Shannon limit. As is shown in FIG. 1, the spectrum efficiency achieved by the third generation W-CDMA system comes near to the Shannon limit. For further spectrum efficiency improvement, the operation point must be moved such that Eb/No increases with increase in R/W in the graph of FIG. 1. However, because of the presence of interference from other cells and sectors in mobile communication environment, there is a limitation by interference (an interference limit curve shown in FIG. 1) in addition to the Shannon limit. It is indicated that the W-CDMA has achieved the spectrum efficiency near to the maximum spectrum efficiency within the limitation of mobile communication system operation defined by both the Shannon limit and the interference limit. In order to achieve further spectrum efficiency improvement, technology for expanding these limits is necessary.
Application of the foregoing array antennas can make the desired radio beam strong against the interference beams and, therefore, this can make the level of the interference limit up. Accordingly, the system operation limitation is expanded. However, as indicated by the Shannon limit curve shown in FIG. 1, R/W sharply rises when Eb/No increases up to 10 dB, but its rise becomes a little as Eb/No further increases from 10 dB to 40 dB. This indicates that powerful action for reducing interference is required to make a great improvement to the spectrum efficiency by the adaptive array antennas.
On the other hand, the foregoing MIMO channels enable configuring a plurality of channels (channel pluralizing) and, therefore, this can expand the Shannon limit of the previous systems using a single channel in the R/W up direction in the graph of FIG. 1. Thus, the effect of improvement to the spectrum efficiency appears to be great. However, because there is also the foregoing interference limit in mobile communication environment, the system operation available range defined by both the interference limit and the Shannon limit is not improved much after all even if the Shannon limit is expanded and the mobile communication system cannot take advantage of the effect using the MIMO channels.